Plywood frames?

Discussion in 'Materials' started by Ian J., Sep 23, 2012.

  1. Ian J.
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    Ian J. Junior Member

    I am planning a building project for this winter of a Crandal Flyer Racing Runabout. Last time I built a wooden boat was a GlenL Squirt some 50 years ago and frames were fitted up from boards cut to shape.

    My question is: I was thinking of building the frames mostly of plywood, and combining the guessets. To the untrained eye, they might appear to be similar to a 'ring' frame but actually two or three components. The first frame, which started my mind to ponder, would be cut completely as one unit with a hole to access the stem. So the rest would follow making the most economical use of a sheet of marine grade fir plywood.

    Your comments please.
     
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  2. rwatson
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    rwatson Senior Member

    Not sure what the question is. Making up a 'ring frame' from multiple overlapping ply sections is a pretty common practice.

    If you use CNC router, with good nesting, you can save a fair bit of ply instead of cutting whole frames.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum Ian.

    Plywood doesn't have the longitudinal strength of solid stock frame futtocks, married with gussets. It's also heavier for the same thickness, so you'll end up with heavier and weaker frames as a result, if using the same dimensions. Usually not a good combination in go fast projects.

    This doesn't mean you can't do it, but that the scantlings will need to be tweaked a tad to accommodate the inherit differences in framing materials. This could be as simple as different frame spacing or different dimensions of the frames themselves or a combination of both.
     
  4. rasorinc
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    rasorinc Senior Member

    Glen-L site does not reccomend plywood for frames. You can look it up on their site.
     
  5. hoytedow
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    hoytedow Carbon Based Life Form

    I built boats with the solid wood long members in the frames with plywood corner braces. Plywood alone is too weak.
     
  6. rwatson
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    rwatson Senior Member

    have you got a link ? I couldn't find it.
     
  7. hoytedow
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    hoytedow Carbon Based Life Form

  8. rasorinc
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    rasorinc Senior Member

  9. Ian J.
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    Ian J. Junior Member

    Thank you gentlemen. That is a lot of information. I could have phrased my thoughts better but ...it was past my bed time and I would have been kept awake thinking, thinking. I do appreciate the concerns with edge grain and this is why there is a big difference with sheet plywood and laminated wood and the functions of a diaphram bulkhead and a ring frame, both of which are technically one piece units.

    For me, it was a side step from building a laminated ring to building a frame out of one or two pieces of sheet plywood and was not able to google anything that said don't and why. I have no qualms about no answers as long as there are qualifying persuasive statements to clarify. I am glad I came here.

    To Mr Watson, hello to from Canada, my wife and I spent two wonderfull weeks on Taz in 2009 for the Hobart Woodenboat Festival and toured the Franklin River Boatbuilding School. We had a great time.

    Ian
     
  10. rwatson
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    rwatson Senior Member

    Glad you had a good time Ian, its Wooden Boat Show again in Jan 2013 ( bi-annual), so preparations are well underway.

    The comment "Some of the plans do incorporate plywood into the frames, but (usually) not as the sole frame element. Plywood's edge won't accept screws that fasten the battens and chines to the frame elements."

    doesnt really equate to "Glen-L site does not recommend plywood for frames."

    My real interest in the subject is a result of a current project that plans to use plywood frames.

    Pars comments (always worth considering) indicate that 'real timber' might be worth incorporating into the plans.
     
  11. PAR
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    PAR Yacht Designer/Builder

    Edge fastening is an issue, but tabbing and epoxy can solve this on some build types. It's not that plywood frames can't be used, but that their good and bad points need to be considered in the scantlings. I spec plywood ring frames frequently on may designs.

    The real concern is the considerable difference in longitudinal strength and stiffness. A hard chine build with relatively straight sided frames, will be lighter and stronger using solid wood futtocks with plywood gussets or lapped and pinned joints. This is because for the same frame spacing, you can use smaller, both molded and sided dimensions, then the same strength and stiffness physical qualities, if built entirely from plywood, plus the edge fastening advantages of solid lumber. The only time this get rivaled is with particularly dense hardwoods, though with these you can use even thinner molded and sided dimensions, because of the great torsional stiffness of solid hardwood stock.

    This is precisely why you see dainty little hardwood frames on relatively close centers in some traditional build types. It's actually lighter and stronger this way, than tripling the spacing, doubling the frame thickness and quadrupling (or considerably more) the molded dimension on a plywood frame.

    From a practical stand point and given the same weight, spacing and approximate thickness at each frame station, the depth of an equivalent plywood frame will be at least 60% more (given sided dimension reductions for weight), so a 1x4 solid wood frame converted to plywood will be at least 6 1/2" deep, which robs usable interior volume, which can be precious in small craft.

    Again you can move the numbers around to help a bit, such as leaving the spacing the same, accepting a weight penalty and reducing the molded dimension a bit to gain reasonable interior volume. Or reducing frame spacing and using similar molded and sided dimensions with an extra stringer or two tossed in, to compensate for stiffness loses. There's several approaches you can take, to incorporate all plywood framing, but you should adjust the scantlings to accommodate the physical properties of the new material choices, if you expect similar strength and stiffness figures for a given weight.
     
  12. alan white
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    alan white Senior Member

    Cost is another issue. Solid wood can be fir or yellow pine or some harder cedars (like spanish cedar) for reasonably light weight frames. Solid wood usually can be had for a fraction of the cost of quality plywood.
    Plywood works well for gussets however---- much better than solid wood where splitting is a problem with short pieces of solid wood. Thickness of gussets can also be thicker to compensate for using a box store grade of plywood without much of a cost penalty (they sell 2 x 4 ft pieces so you don't have to buy a whole sheet.
    Plywood frames eat up plywood quickly as well because of the vast amount of waste while solid wood wastes very little. Cutting of solid wood is reduced when the design is for a plywood hull too.
    I think a case can be made for plywood frames when building a round-chined design such as would be built stripper style. Stress isn't localized so much and it's more work to cut seperate solid wood futtocks on curves.
    I'm assuming you're building a plywood hull. For cost, labor, and weight in the final product, solid wood seems to me to be the best way to go,
     
  13. PAR
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    PAR Yacht Designer/Builder

    My recollection of the boat he's building is the 15' design by Bruce Crandall. Is an old design, one of the C class racers, a warped bottom with an odd step, though typical of it's era. I'm fairly sure it's one of the "build a boat" series that ran in MoToR BoatinG magazine back in the 1930's. The original versions of this particular boat liked to trip over it's chine and this boat was an improvement with beveled chines. It was faster and safer as a result. The lines shown are the first generation with the hard chine line.

    Compared to modern designs they pound, are heavy and have high speed handling issues, if over powered. They're often wet rides, but this isn't much of a consideration for a racer and they look good with nicely tumblehome sterns. It's one of the "aero" designs that dominated the styling of the era.

    [​IMG]

    [​IMG]

    I'll assume you're getting plans from Classic Wooden Boat Plans.com
     
  14. Ian J.
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    Ian J. Junior Member

    Nope, not that one. Its an O/B version of an earlier design without the stepped bottom. I have the volume of MotorBoating with the write up. Bruce Crandall revised it yet again with the hull planked in plywood, and the bottom was laid with stepped layers of plywood to provide longitudinal strakes. The earlier version had a little deadrise all the way to the transom and the newer one was flat. Thats ok for flat water, I prefer the earlier one for salt water running.

    Plywood sheeting as a diaphram bulkhead is out, but still might think of a laminated ring with filler blocks at the corners. That way I would have the bite for screws and no extreme amounts of exposed endgrain. I went back to a book of cold molding by Ian Nicholson and started reading it again. Dont ask me why I am focused on alternatives, I just like be different. Thinking outside the herd.
     

  15. Ian J.
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    Ian J. Junior Member

    A reprint of the Motor Boating text is available from DN Goodchild #5385 for about $8
     

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